Strömungsparameter

Strömungsparameter is a term used in fluid dynamics to describe dimensionless numbers that characterize fluid flow. These parameters arise when the governing equations of motion are nondimensionalized and express the relative importance of different physical effects, such as inertia, viscosity, gravity, compressibility, surface tension, and diffusion. They provide a compact way to compare different flow situations and to scale laboratory experiments to real-world applications.

Common examples and meanings include:

- Reynolds number Re = rho U L / mu, ratio of inertial to viscous forces, indicating whether flow is laminar or turbulent for a given situation.

- Froude number Fr = U / sqrt(g L), ratio of inertial to gravitational forces, relevant for open-channel and free-surface flows.

- Mach number Ma = U / c, ratio of flow speed to the speed of sound, governing compressibility effects.

- Weber number We = rho U^2 L / sigma, ratio of inertial to surface-tension forces, important in droplets and interfaces.

- Capillary number Ca = mu U / sigma, ratio of viscous to surface-tension forces.

- Strouhal number St = f L / U, nondimensional frequency in oscillating or shedding flows.

- Péclet number Pe = U L / alpha, ratio of advective to diffusive transport.

Derivation typically follows nondimensionalization of the Navier–Stokes equations, using characteristic length L and velocity U. The resulting dimensionless groups help identify dominant physics, inform model testing, and enable similarity between experiments and real systems. They are problem dependent; choosing L and U is not unique, and several parameters may be relevant simultaneously. The concept is rooted in the Buckingham Pi theorem, which explains how many and which independent dimensionless groups arise from the governing equations.